1. Field of the Invention
The present invention relates to a multi-beam optical scanning apparatus and an image forming apparatus using the multi-beam optical scanning apparatus. In particular, the present invention relates to a technique suited for an image forming apparatus, such as a laser beam printer using an electrophotographic process, a digital copying machine, or a multifunctional printer (versatile printer), where image information is recorded by deflecting multiple light beams emitted from light source means having multiple light emission portions using a rotation polygon mirror (polygon mirror) serving as a light deflector and then optically scanning a surface to be scanned through an imaging optical system having fθ characteristics (imaging optical system).
2. Related Background Art
Conventionally, in an optical scanning apparatus, a light flux (light beam) optically modulated and emitted from light source means in accordance with an image signal, then the light beam is periodically deflecting by a light deflector composed of a rotation polygon mirror or the like, the deflected light beam is condensed in a spot manner on a photosensitive recording medium surface by an imaging optical system having fθ characteristics, and the surface is scanned with the condensed light beam, to thereby record an image.
In recent years, along with speedup and miniaturization of an apparatus such as a laser beam printer, a digital copying machine, or a multifunctional printer, further speedup and miniaturization of an imaging optical system used as an optical system have been desired.
As a method of achieving the speedup, an overfilled optical system is used, for instance.
In the overfilled optical system (hereinafter referred to as the “OFS”), it is sufficient that a deflecting surface (reflecting surface) of a rotation polygon mirror covers a light beam width substantially required for deflection and scanning in a wide width of an incident light beam, which leads to a feature that it is possible to increase the number of surfaces of the rotation polygon mirror with a small diameter, which is suited for the speedup.
In the OFS, the incident light beam onto the rotation polygon mirror is Gaussian-distributed so that its light intensity shows the maximum in proximity to the optical axis of a condensing optical system, and a reflecting and deflecting region changes along with a field angle from the vicinity of the optical axis to an end portion, so there is a tendency that illuminance on a surface to be scanned is decreased as an image height is increased.
In addition, a fact that the light beam width in the main scanning direction of the reflected and deflected light beam decreases as the field angle increases in the OFS strengthens the tendency that the illuminance on the surface to be scanned decreases as the image height increases. An illuminance distribution on the surface to be scanned in this case is shown in FIG. 12. As can be seen from FIG. 12, in an imaging optical system using the OFS, there is a tendency that the illuminance on the surface to be scanned decreases as the image height increases (hereinafter referred to as the “light amount decreasing”).
In addition, an illuminance distribution on a surface to be scanned in a case where a position, at which a light intensity of an incident light beam is the maximum on a deflecting surface, is shifted from a center of an effective light beam width in the main scanning direction of the deflecting surface to an end of the effective light beam width due to an error of installation of light source means or the like is shown in FIG. 13. As can be seen from FIG. 13, in addition to the light amount decreasing, there is a tendency that the illuminance on the surface to be scanned increases and decreases along with changes of the image height (from −Y0 to +Y0).
As described above, unevenness in illuminance occurs in one scanning line on a surface to be scanned, which results in a problem that when the OFS is applied to an image forming apparatus, unevenness in density of image occurs to a formed image.
In addition, in the case of a multi-beam OFS where the OFS is used as a means for achieving further speedup and simultaneously scans a surface to be scanned with light beams from multiple light emission portions onto the surface to be scanned so that the light beams are arranged side by side in a direction perpendicular-to a scanning direction, at the time of formation of images having the same pattern, in addition to nonuniformity in illuminance in one scanning line on a surface to be scanned, an illuminance difference (hereinafter referred to as the “inter-beams illuminance difference”) occurs among multiple beams at the same image height, so there is a problem in that unevenness in density of image occurs to a formed image.
Hereinafter, a principle of occurrence of an illuminance difference between two light beams at the same image height will be described.
A multi-beam optical scanning apparatus is a system to speed up recording of image information by simultaneously scanning a surface to be scanned with multiple light beams emitted from light source means including multiple light emission portions onto the surface to be scanned. As an example of the light source means used in this case, a multi-beam semiconductor laser will be described with reference to FIG. 14. FIG. 14 is a main portion schematic diagram of a monolithic multi-beam semiconductor laser including two light emission portions (lasers) 14a and 14b. Note that the light emission portion 14a will also be referred to as the “laser A” (or the “light emission portion A”) and the light emission portion 14b will also be referred to as the “laser B” (or the “light emission portion B”).
As shown in FIG. 14, a multi-beam semiconductor laser 1 includes two light emission portions 14a and 14b on one active layer 13 and a divergent light beam is emitted from each of the two light emission portions 14a and 14b. The intensity of the divergent light beam is in Gaussian-distribution so that the light intensity is the maximum at its corresponding intensity center line 14ap or 14bp. As shown in FIG. 14, in the actually manufactured multi-beam semiconductor laser 1, a difference exists between directions of the intensity center lines 14ap and 14bp of the light beams emitted from the two light emission portions 14a and 14b and an angle difference RΔθ in the main scanning direction occurs between the intensity center lines 14ap and 14bp of the light beams emitted from the two light emission portions 14a and 14b. 
FIG. 15 is a main portion sectional view (main scanning sectional view) taken in a main scanning direction of a multi-OFS where the multi-beam semiconductor laser shown in FIG. 14 is used as light source means.
In the drawing, an incident optical system is illustrated as one condensing optical system 11. Also, in the drawing, an angle difference between intensity center lines 14ap and 14bp of two light beams in the main scanning direction is illustrated as RΔθ.
In the drawing, at the time of passage of the two light beams through the condensing optical system 11, intensity center lines 14ap and 14bp of the two light beams having the angle difference RΔθ pass at different heights from an optical axis 12 of the condensing optical system 11, so an interval di with respect to the main scanning direction occurs between the intensity center positions 14ap and 14bp of the two light beams in proximity to a deflecting surface 7.
As a result, the two light beams incident on the deflecting surface 7 have mutually different intensity distributions in the main scanning direction, so the illuminance distributions of the two light beams on the surface to be scanned with respect to an image height also differ from each other. This is shown in FIG. 16.
FIG. 16 illustrates illuminance distributions 14ai and 14bi of the respective light beams emitted from the two light emission portions 14a and 14bi n the case where the amounts of the lights emitted from the two light emission portions 14a and 14b are controlled so as to be equal to each other and constant at every image height for the sake of formation of images having the same pattern. It can be seen from the drawing that in addition to the tendency that the illuminance on the surface to be scanned decreases as the image height increases, an illuminance difference occurs between the two light beams at the same image height.
Various multi-beam optical scanning apparatuses and optical scanning apparatuses solving this problem are proposed (see Japanese Patent Application Laid-Open Nos. H11-218702 and H09-197316).
In Japanese Patent Application Laid-Open No. H11-218702, a configuration is disclosed in which illuminance in one scanning line on a surface to be scanned is made approximately uniform by configuring an opening shape of an opening plate, disposed in an optical path between light source means and deflecting means, wider in a sub-scanning direction than a center portion in a main scanning direction.
In Japanese Patent Application Laid-Open No. H09-197316, a configuration is disclosed in which illuminance in one scanning line on a surface to be scanned is made approximately uniform by supplying a semiconductor laser drive current based on a correction coefficient in accordance with a scanning position so as to approximately uniformize the illuminance in one scanning line on the surface to be scanned.
However, in the multi-beam optical scanning apparatuses proposed in Japanese Patent Application Laid-Open Nos. H11-218702 and H09-197316, it is difficult to reduce the inter-light-beams illuminance difference.
For instance, in Japanese Patent Application Laid-Open No. H11-218702, a configuration is disclosed in which nonuniformity in a light amount distribution is suppressed through adjustment of the opening shape of the opening plate with respect to one light beam. FIG. 17 shows an illuminance distribution in the case where the technique disclosed in Japanese Patent Application Laid-Open No. H11-218702 is applied to a multi-OFS, such as the OFS shown in FIG. 15, in which intensity center lines of two light beams have an angle difference RΔθ in a main scanning direction. As can be seen from FIG. 17, although the light amount decreasing of both of the two light beams reduces by passing of the two light beams through the opening plate having the opening shape whose width in the sub-scanning direction is wider than the center portion in the main scanning direction, it is impossible to suppress a light amount difference between the two light beams at the same image height.
Japanese Patent Application Laid-Open No. H09-197316 discloses an embodiment in which an illuminance distribution in one scanning line on a surface to be scanned of one laser is detected to calculate a correction coefficient to uniformly correct illuminance in one scanning line, and every laser drive current is supplied based on the correction coefficient illuminance distributions 14ai and 14bi are shown in FIG. 18, in the case where with respect to a multi-OFS, such as the OFS shown in FIG. 15, in which intensity center lines 14ap and 14bp of two light beams has an angle difference RΔθ in the main scanning direction, an illuminance distribution in one scanning line on a surface to be scanned of a laser A (light emission portion 14a) is detected, then a correction coefficient to uniformly correct illuminance in one scanning line is calculated, and laser A and B drive currents are supplied based on the correction coefficient. As can be seen from FIG. 18, the same correction coefficient is used for multiple light beams, so although the light amount distribution 14ai of the laser A can be corrected uniformly, the light amount distribution 14bi of the laser B (light emission portion 14b) has a gradient, which makes it impossible to suppress an illuminance difference between multiple light beams.
Unevenness in density of image due to such an inter-light-beams illuminance difference where there are differences in density between multiple dots adjacent to each other is more conspicuous than unevenness in density of image due to light amount decreasing where density continuously changes in one scanning line. Therefore, even if the light amount decreasing is completely eliminated, when the inter-light-beams illuminance difference remains, it is impossible to form a favorable image where no unevenness in density of image exists.